Post-only restricts a limit order to entering the order book as resting liquidity. Under a common implementation, if the order would immediately match an existing order, the venue rejects or cancels it instead of allowing taker execution.
Price and timing matter
Suppose the best ask is $10. A post-only buy limit at $9.99 can rest if no eligible sell order crosses that price when it reaches the matching engine. A buy limit at $10 would immediately match the ask and therefore be canceled or rejected under this implementation.
Even a price that looked nonmarketable on the screen can cross by the time the request arrives. Confirm the resulting status instead of assuming the order was posted.
Maker status is not a fill guarantee
Post-only can help a trader control whether an execution qualifies as maker activity. The actual fee or rebate depends on the venue, product, and fee tier; maker does not universally mean free.
An accepted order may never fill, and a resting quote can be executed just as the market moves unfavorably. Post-only does not automatically follow the best price or guarantee that a canceled order is resubmitted.